1. Field of the Invention
Embodiments of the present invention relate generally to the packaging of semiconductor dice. More particularly, embodiments of the present invention relate to methods and apparatus for redirecting a flow of a molding compound, such as a resin, in a mold cavity, and resulting semiconductor device packages.
2. State of the Art
Semiconductor dice are created from wafers, such as silicon wafers, as well as from other bulk semiconductor substrates, using a sequence of material deposition and removal acts that are well known to those of ordinary skill in the art. Ultimately, the completed semiconductor dice may be packaged for many applications by encapsulating one or more semiconductor dice in a resin-based molding compound comprising, conventionally, a silicon particle-filled thermoplastic resin.
One approach to packaging of semiconductor dice utilizes a lead frame. Lead frames are generally thin metallic layers, although metal-coated polymer film lead frames are also known. In the center of some lead frames is a so-called “die paddle” upon which one or more semiconductor dice may be mounted, such as by an adhesive or a solder. Lead fingers, or leads, are disposed adjacent one or more sides of the die paddle. The paddle is isolated from the remainder of the lead frame except by members called tie bars, which are used to initially support the lead frame when in strip form with other identical lead frames, before encapsulation of the die or dice and a subsequent trim and form operation wherein the lead frame lead fingers as well as the die paddle are severed from a surrounding, supporting structure. The lead fingers, when a lead frame is in strip form and during encapsulation of a semiconductor die or dice carried thereon, are separated from each other by intervening, supporting dam bars, which are also severed during the trim and form operation.
Once a semiconductor die is secured to the die paddle and wire-bonded or otherwise electrically connected to the lead fingers, each resulting assembly, in strip form with a plurality of identical assemblies, is then placed in a mold cavity of a plurality of mold cavities defined between opposing segments of a mold of a transfer molding apparatus. Molten molding compound, comprising the aforementioned silicon particle-filled thermoplastic resin, is injected under pressure into the mold cavity to encapsulate and form a package around the semiconductor die and the plurality of lead fingers. The packaged semiconductor die is removed from the mold and separated from the lead frame in the aforementioned trim and form operation, wherein the lead fingers may also be conventionally formed into a final configuration to facilitate connection of the outer ends thereof to higher-level packaging. The packaged semiconductor die is then available for use, such as in mounting to a printed circuit board (PCB) or to other higher-level packaging.
Other lead frame configurations, such as so-called leads-over-chip (LOC) configurations, wherein no die paddle is employed and lead fingers of a lead frame are adhered to an active surface of a semiconductor die with inner ends thereof proximate a central row or rows of bond pads. A similar configuration, known as a lead-under-chip (LUC) configuration, likewise does not employ a die paddle, and the lead fingers extend under and are adhered to a back side of a semiconductor die. Another configuration is a so-called “leads between chip” configuration, wherein a lead frame (with or without paddle) is interposed between semiconductor dice on opposing sides thereof. The encapsulation process, followed by trim and form, is generally the same for these semiconductor device assembly configurations as for die paddle-type assemblies.
A problem with current packaging processes is that mold cavity configurations and tolerance variances, semiconductor die shapes and sizes, the presence of more than one semiconductor die to be encapsulated, and the configuration and orientation of the lead frame elements may lead to uneven flow of resin inside a mold cavity, resulting in an uneven distribution of resin around the semiconductor die or dice and the lead frame in the mold cavity. The resulting voids, knit lines, and pinholes in the resin encapsulant structure can compromise the integrity of the packaging around the semiconductor die or dice and can also adversely affect heat transfer characteristics of the package.
To further explain how the aforementioned defects may occur, a conventional mold comprises a plurality of mold cavities defined between a top mold plate and a bottom mold plate. The semiconductor dice assembled with, and electrically connected to, lead frames carried by a supporting structure of a lead frame strip, are disposed in the mold cavities with the supporting structure outside the cavities and connected to the assemblies by tie bars and lead fingers. The molding compound is introduced into each mold cavity through one or more “gates,” or openings, leading to the cavity, generally from one side of the mold cavity and displacing air in the mold cavity out through one or more apertures, also termed vents, in the opposing side of the cavity, although vertically oriented mold cavities are known. In any case, the aforementioned variables in the mold cavities as well as in the semiconductor die assemblies placed therein may cause the flow front of the pressurized molding compound to accelerate around the sides of each assembly, particularly if the assemblies include a stack of semiconductor dice, leaving the top and bottom of the stacks inadequately encapsulated. This phenomenon is due to the relative difference in resistance to molding compound flow provided by the relatively larger cross-sectional areas to the sides of the die stack provided by the mold cavity in comparison to the smaller cross-sectional areas, above and below, between the stack and the mold cavity walls.
Additionally, semiconductor dice and their corresponding connections with the lead frames may be modified over time. A mold cavity may have been optimally designed to avoid the formation of voids in an initial lead frame configuration of a package for a certain semiconductor die or die stack. However, when the semiconductor die is modified such that the size or shape of the die has changed (for example, when die “shrinks” are implemented from one generation of a die to the next, to increase yield per wafer), then voids may form in the encapsulant due to a change in the flow pattern thereof during transfer molding. As retooling the mold plates may cost well in excess of one hundred thousand dollars, it would be desirable to be able to beneficially modify the flow characteristics of resin within a mold cavity without having to modify the mold cavity itself. Several attempts to solve this problem by modifying a lead frame characteristic have been made.
One attempt to control the flow of resin in a mold cavity has been to kink, cut, or bend a tie bar adjacent to a gate of the mold cavity. Another approach has been to form an offset in a lead frame, such as in a tie bar, close to a gate to affect the flow of resin entering the mold cavity. Both approaches are limited to controlling the amount of resin that flows across the top or bottom surface of a lead frame. Additionally, both approaches are limited to being located near a gate of a mold cavity.
A third approach has been to include an additional hole in a lead frame to allow resin to flow into a top portion of a mold cavity before flowing into a bottom portion. However, this approach is unable to modify a flow of resin within the cavity once the resin has left the gate area of the mold cavity.
A fourth approach has been to extend a region of a lead finger of a lead frame, where the extended region is in the same plane as the lead finger and the remainder of the lead frame. The extended region is close to the gate of the mold cavity and purportedly results in a more even distribution of resin across the top and bottom of the lead frame. This approach, again, requires that the extended region must be close to the gate of the mold cavity.
A fifth approach has been to modify the flow of resin on a lead finger-by-lead finger basis. In this approach, the ends of the lead finger may be down-set similar to the down-set of the paddle of a lead frame. A lead finger may then be wire-bonded to the semiconductor die. The resulting vertically oriented portion of the lead finger purportedly reduces the flow rate of resin right at the bonding point of a wire to the end of the lead finger. The fifth approach also involves forming a second wire ball to cover the heel of the wire bond connecting a lead finger and a semiconductor die. A second wire is attached to the second wire ball and to the lead finger. The second wire and the second ball serve to retard the flow of resin at the lead finger wire bonding point. The fifth approach is generally not viable for solving the problem of voids, pinholes, and knit lines in a package.
A sixth approach includes the use of a flow diverter positioned adjacent a flow hole in a lead frame. The flow diverter is positioned to increase the volume of material that passes through the flow hole and underneath the lead frame.
A need exists in the art for a lead frame that may be modified in a wide variety of locations to restrict and redirect the flow of resin within a mold cavity. A further need exists to control the flow rate of resin around the sides of a semiconductor die stack.
Heretofore, the packaging of semiconductor dice has been discussed in the context of lead frame-mounted dice. However, some semiconductor dice or chips are packaged after mounting the chip onto a substrate, such as a printed circuit board (PCB). For example, with so-called “flip-chips,” the active surface of the semiconductor die bears solder balls or other discrete conductive elements protruding therefrom, which serve to mechanically and electrically connect the semiconductor die to a supporting substrate. A flip-chip may be underfilled with a flowable dielectric material to surround the space between and around the discrete conductive elements. The flip-chip may then be wholly or partially encapsulated in a packaging resin, or the encapsulation performed concurrently with the underfill.
Additionally, semiconductor dice may be mounted directly to substrates other than lead frames, such as to interposer substrates in a chip-on-board (COB) or board-on-chip (BOC) structure, which may be configured as, for example, ball grid array (BGA), pin grid array (PGA) or land grid array (LGA) packages. For example, a number of semiconductor dice may be mounted to an array of unsingulated interposer substrates disposed within a mold cavity, and then resin flowed over the surface of the interposer substrates to encapsulate the dice and conductive elements, such as wire bonds, electrically connecting the semiconductor dice to the interposer substrates. Once the resin is cured, the individual encapsulated semiconductor dice mounted to their respective interposer substrates may be separated, or “singulated,” from each other.
Generally, whenever a molding compound such as a thermoplastic resin or other flowable dielectric material is forced to flow over or around a lead frame or other substrate with one or more semiconductor dice mounted thereto within a mold cavity, the resin may flow faster around the sides of the dice than over and under the dice. This may result in voids forming over the tops and bottoms of some of the semiconductor dice, particularly the semiconductor dice in a mold cavity farthest from the resin entry point. A need exists in the art to reduce the flow of resin in between the dice to avoid the formation of voids and other defects in the packaging encapsulant.